Sulphur power

Sulphur is an amazingly versatile element, as this blog never tires of telling people. Fellow poster Thiophilos reminds people of sulphur’s virtues as a power source, utilising the heat of combustion in electricity generation, as happens daily at countless sulphur-burning sulphuric acid plants around the world.

But sulphur is now also set to make other inroads into powering our lives, this time via battery technology. As the number and capability of portable electronic devices continues to increase, our appetite for portable power sources is also growing, and at a staggering rate. Battery technology has already had several step changes over the past few decades, from lead-acid through nickel-cadmium and alkaline cells, to lithium ion batteries, which are the new workhorses of the consumer electronics industry, especially for rechargeable applications like mobile phones, tablets and laptop computers. But the need to increase energy density continues, especially if we are to move to electrically-powered vehicles.

One of the problems is with the cathode of lithium ion batteries, which in the case of lithium oxides or lithium iron phosphate might only have fraction of the specific charge capacity of the silicon or graphite anode. Over the past few years, the focus of attention has turned to sulphur as a cathode – sulphur has a very high theoretical specific capacity. As a result, and also because of the relatively lightweight elements that constitute them, lithium sulphur (Li-S) batteries can have two to four times the energy density of lithium-ion batteries, both in weight and volume terms. However, one of the problems with using sulphur is its poor conductivity, and another is that it tends to form lithium polysulphides which are soluble in the battery electrolyte. This means that the Li-S batteries’ ability to be discharged and recharged rapidly degrades away over several charging cycles.

Commercial Li-S batteries have been under development by several companies, using coated sulphur and electrolytes which minimise the dissolution of the lithium polysulphides, but in the meantime work has been under way in research institutes around the world to completely overcome the issues with Li-S batteries, and now Dr Hailiang Wang and his team at Stanford University in the US say that they may have found a way. The technique that they use is a polyethylene glycol (PEG) coating on sub-micron particles of sulphur. The PEG coating traps polysulphides and prevents them dissolving away. Nanoengineering techniques are then use to wrap the coated particles in a ‘cage’ of graphene molecules. The interaction between the carbon and sulphur renders the particles electrically conducting, and also supports them as they swell and shrink during each charging cycle.

As the paper says: “it is worth noting that the graphene-sulphur composite could be coupled with silicon-based anode materials for rechargeable batteries with significantly higher energy density than currently possible.”

There is clearly a lot of work still to be done before such a technique can be first optimised and then performed on an industrial scale. However, in the meantime the first generation of lithium-sulphur batteries are already being used in demonstration applications. One was used in the world’s longest unmanned flight, using a solar-powered aircraft which stayed aloft for 14 days in July 2010. Companies like Oxis Energy and Sion Power are racing to commercialise Li-S batteries and within a few years it may well be that your mobile phone or tablet computer, and possibly even your car, will be powered by sulphur.